refactor: standardize config structure and consolidate modules

- Standardized apps/ directory by renaming all entry points (e.g., home.nix) to default.nix and updating imports to use directory paths.
- Consolidated system/ logic into modules/system/, eliminating the top-level system/ directory and redundant wrappers.
- Merged subsidiary utility scripts (e.g., hakase-popup.nix, switch-wallpaper.nix) into their parent default.nix files for better cohesion.
- Cleaned up unused files and updated all module references to reflect the new structure.

modules/system/security.nix

@@ -0,0 +1,31 @@
+{
+  pkgs,
+  myConfig,
+  config,
+  ...
+}: {
+  security.pam.services.greetd.enableGnomeKeyring = true;
+  environment.systemPackages = with pkgs; [
+    sops
+    age
+    ssh-to-age
+  ];
+  sops = {
+    defaultSopsFile = ../secrets/secrets.yaml;
+    age.keyFile = "/home/${myConfig.nixos.username}/.config/sops/age/keys.txt";
+    secrets = {
+      default_password = {
+        neededForUsers = true;
+      };
+
+      AVANTE_GEMINI_API_KEY = {
+        owner = "kenji";
+      };
+    };
+  };
+  programs.fish.interactiveShellInit = ''
+    if test -f ${config.sops.secrets.AVANTE_GEMINI_API_KEY.path}
+      set -gx AVANTE_GEMINI_API_KEY (cat ${config.sops.secrets.AVANTE_GEMINI_API_KEY.path})
+    end
+  '';
+}